C64_SKILLS/.ai/skills/vic-ii-graphics/SKILL.md

Skill: VIC-II Graphics, Screen, Characters, Sprites, and Raster

Use this skill when

The task involves screen memory, colors, sprites, character sets, bitmap graphics, raster interrupts, scrolling, border/background, or VIC-II banking.

Core registers

VIC-II base: $D000 / 53248.

Decimal	Hex	Purpose
53248-53263	$D000-$D00F	Sprite X/Y coordinates
53264	$D010	High bits for sprite X positions
53265	$D011	Control register 1; vertical scroll, screen on, bitmap, raster bit 8
53266	$D012	Raster line low byte
53267	$D013	Light pen X
53268	$D014	Light pen Y
53269	$D015	Sprite enable bits
53270	$D016	Control register 2; horizontal scroll, multicolor
53271	$D017	Sprite Y expansion
53272	$D018	Memory control: screen and character/bitmap pointers
53273	$D019	Interrupt flags
53274	$D01A	Interrupt enable
53275	$D01B	Sprite/background priority
53276	$D01C	Sprite multicolor enable
53277	$D01D	Sprite X expansion
53278	$D01E	Sprite-sprite collision
53279	$D01F	Sprite-background collision
53280	$D020	Border color
53281	$D021	Background color 0
53282-53284	$D022-$D024	Extra background/multicolor registers
53285-53286	$D025-$D026	Sprite multicolor registers
53287-53294	$D027-$D02E	Sprite individual colors

Screen basics

Default screen RAM: $0400 / 1024. Default color RAM: $D800 / 55296. Screen is 40 columns x 25 rows = 1000 character cells.

Cell address:

screen = 1024 + row * 40 + column
color  = 55296 + row * 40 + column

Sprite basics

• Each sprite is 24 x 21 pixels.
• Each sprite pattern uses 64 bytes.
• Default sprite pointers are $07F8-$07FF / 2040-2047.
• Pointer value = sprite data address / 64, within the active VIC bank.
• Enable sprites with $D015 bits.

Example enable sprite 0 in BASIC:

POKE 53269,PEEK(53269) OR 1

VIC-II bank rule

The VIC-II sees one 16K bank at a time. CIA #2 port A controls the video bank. All active screen RAM, character data, bitmap data, and sprite data must be visible inside the selected VIC bank.

Character ROM copy warning

To copy character ROM from CPU-visible $D000-$DFFF, you must bank character ROM in and I/O out. Since I/O disappears during that window, disable interrupts briefly and restore banking immediately.

Raster interrupt principles

• Set raster line in $D012; bit 8 is in $D011.
• Enable raster IRQ in $D01A bit 0.
• Acknowledge VIC interrupt by writing the relevant bit back to $D019.
• Chain or jump to the normal KERNAL IRQ handler unless replacing the system IRQ completely.

Agent checklist

1. Give addresses in decimal and hex.
2. Preserve unrelated VIC register bits.
3. Note PAL/NTSC timing differences for raster work.
4. Keep sprite data inside active VIC bank.
5. For character/bitmap changes, explain $D018 layout.